2. SEISMIC INTERPRETATION
Seismic Interpretation is the extraction of subsurface geologic
information from seismic data. It is correlating the features we see
in seismic data with elements of geology as we know them.
OBJECTIVE:-
Prospect generation and identification of suitable locations for drilling by
interpreting subsurface geo-data. The primary goal of seismic interpretation
is to make maps that provide geologic information (reservoir depth structure,
thickness, porosity, saturation, etc.).
3. To start interpretation, We must have:
• Base Map: shot point location
• Seismic sections: Inline & Crossline
• Available Wells
• Velocity data from wells: from Cheak Shot,
VSP.
• Formation Top of the well: to determine the
top of horizon
• Logs & reports: Sonic, GR, Density & other
logs
5. 1-Loading the data:
• Seismic sections: (post-stack data).
• Available Wells data: Well logs & formation tops
• Velocity Data of wells: from Check-shot survey or
Vertical Seismic Profiling.
12. • A type of borehole seismic data designed
to measure the seismic travel-time from the
surface to a known depth. P-wave velocity of
the formations encountered in a wellbore can
be measured directly by lowering a
geophone to each formation of interest,
sending out a source of energy from the
surface of the Earth, and recording the
resultant signal. From this survey, we will
have velocity & depth, we estimate the time
& plot result in Time/Depth Scale. At this
case, the reading will be taken at every 100ft
Check Shot survey:
13. VSP refers to measurements made in a
vertical wellbore using geophones inside
the wellbore and a source at the surface
near the well. Most VSPs use a surface
seismic source, which is commonly a
vibrator on land and an air gun in offshore
or marine environments. Another
advantage for VSP is the ability to give
good results in deviated wells, where
synthetic Seismogram are often
unreliable. VSP produces Time/Depth
Scale & VSP image, where Check-shot just
produces time/depth scale. VSP has
higher resolution than Check-shot survey (
reading every 10ft)
Vertical
Seismic
Profiling
(VSP):
14. There are many types of VSP survey:
• Zero-offset VSP: in which the energy source is positioned directly above the receivers, typically very close
to the wellbore.
• Offset VSP: in which the source is located at an offset from the drilling rig during acquisition. This allows
imaging to some distance away from the wellbore.
• Movable source VSP: in this case the source is not stationary.
• Walk-away VSP: in which the source is moved to progressively farther offset at the surface and receivers
are held in a fixed location, effectively providing a mini 2D seismic line that can be of higher resolution
than surface seismic data and provides more continuous coverage than an offset VSP
• Walk-in VSP: originating from successive shots fired from far offset source with decreasing offset.
• Walk-above VSP: accommodate the geometry of a deviated well; sometimes called a vertical incidence
VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases.
Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well.
15.
16. 2-Well Tie
• We create a Synthetic Seismogram to know the accurate location of the formation tops of
inter-seated horizon then tie it with the seismic section. Synthetic indicates also that if the
horizon response is peak or trough. From the well, we know the depth of the event (Formation
tops). From plotting values of depths & times which came from the check-shot survey, we can
extract the time value for certain depth ( to mark that depth on seismic section). We repeat
these steps with all wells to get the true depth of the horizon.
• We must know the datum of survey (datum survey in seismic called Seismic reference
datum).
• If the Check-shot time is one way time, we must convert it to two way time.
• We must know the type of well depth (TVD, MD, or TVD subsea).
